The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics of light weight, high strength, toughness, thermal resistance, and ability to being formed and shaped can be used to great advantage. Such components are used, for example, in aeronautical, aerospace, satellite, high performance recreational products, and other applications.
Typically, such components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid, polyester and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure.
Through the use of such reinforcement materials, which ultimately become a constituent element of the completed component, the desirable characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The typical constituent reinforcement materials may be woven, knitted or otherwise oriented into desired configurations and shapes for reinforcement preforms. Usually particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected. Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components.
After the desired reinforcement preform has been constructed, matrix material may be introduced to and into the preform, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the interstitial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical thermal or other properties. Typically, however, they will not be of the same materials or have comparable physical, chemical, thermal, or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified masses of the matrix material normally are very strongly adhered to the reinforcing material (e.g., the reinforcement preform).
As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcement preform. Any break or discontinuity in the reinforcement preform limits the ability of the preform to transfer and bear the stress applied to the finished component.
Frequently, it is desired to produce components in configurations that are other than such simple geometric shapes as, for example, plates, sheets, rectangular or square solids, etc. A way to do this is to combine such basic geometric shapes into the desired more complex forms. One such typical combination is made by joining reinforcement preforms made as described above at an angle (typically a right-angle) with respect to each, other. Usual purposes for such angular arrangements of joined reinforcement preforms are to create a desired shape to form a reinforcement preform that includes one or more end walls or “T” intersections for example, or to strengthen the resulting combination of reinforcement preforms and the composite structure which it produces against deflection or failure upon it being exposed to exterior forces, such as pressure or tension. In any case, a related consideration is to make each juncture between the constituent components as strong as possible. Given the desired very high strength of the reinforcement preform constituents per se, weakness of the juncture becomes, effectively, a “weak link” in a structural “chain.”
An example of an intersecting configuration is set forth in U.S. Pat. No. 6,103,337, the disclosure of which is incorporated herein by reference. This reference sets forth an effective means of joining together two reinforcing plates into a T-form. This can be accomplished by joining a first reinforcing panel to a second reinforcing panel placed on edge against the first panel.
Various other proposals have been made in the past for making such junctures. It has been proposed to form and cure a panel element and an angled stiffening element separate from each other, with the latter having a single panel contact surface or being bifurcated at one end to form two divergent, co-planar panel contact surfaces. The two components are then joined by adhesively bonding the panel contact surface(s) of the stiffening element to a contact surface of the other component using thermosetting adhesive or other adhesive material. However, when tension is applied to the cured panel or the skin of the composite structure, loads at unacceptably low values resulted in “peel” forces which separate the stiffening element from the panel at their interface since the effective strength of the joint is that of the adhesive and not that of the matrix or the reinforcement materials.
The use of metal bolts or rivets at the interface of such components is unacceptable because such additions at least partially destroy and weaken the integrity of composite structures themselves, add weight, and introduce differences in the coefficient of thermal expansion as between such elements and the surrounding material.
Other approaches to solving this problem have been based on the concept of introducing high strength fibers across the joint area through the use of such methods as stitching one of the components to the other and relying upon the stitching thread to introduce such strengthening fibers into and across the juncture site. One such approach is shown in U.S. Pat. No. 4,331,495 and its method divisional counterpart, U.S. Pat. No. 4,256,790. These patents disclose junctures having been made between a first and second composite panel made from adhesively bonded fiber plies. The first panel is bifurcated at one end to form two divergent, co-planar panel contact surfaces in the prior art manner, that have been joined to the second panel by stitches of uncured flexible composite thread through both panels. The panels and thread have then been cured simultaneously or “co-cured.” Another method to improve upon junction strength is set forth in U.S. Pat. No. 5,429,853.
While the prior art has sought to improve upon the structural integrity of the reinforced composite and has achieved success, particularly in the case of U.S. Pat. No. 6,103,337, a desire exists to improve thereon or address the problem through an approach different from the use of adhesives or mechanical coupling. In this regard, one approach might be to create a woven three dimensional (“3D”) structure by specialized machines. However, the expense involved is considerable and rarely is it desirable to have a weaving machine directed to creating a single structure. Despite this fact, 3D preforms which can be processed into fiber reinforced composite components are desirable because they provide increased strength relative to conventional two dimensional laminated composites. These preforms are particularly useful in applications that require the composite to carry out-of-plane loads. However, the prior-art preforms discussed above have been limited in their ability to withstand high out-of-plane loads, to be woven in an automated loom process, and in some cases, to provide for varying thickness of portions of the preform.
Another approach would be to weave a two dimensional (“2D”) structure and fold it into a 3D shape. Early attempts at folding 2D preforms into 3D shapes typically resulted in parts that distort when the preform was folded. The distortion occurs because the lengths of fiber as-woven are different than what they should be when the preform is folded. This causes dimples and ripples in areas where the as-woven fiber lengths are too short, and buckles in the areas where fiber lengths are too long. An example of a 3D preform weave architecture, which may lead to ripples or loops in areas where the preform is folded, is disclosed in U.S. Pat. No. 6,874,543, the entire content of which is incorporated herein by reference.
One approach to solve the problem of distortion upon folding is disclosed in U.S. Pat. No. 6,446,675, the entire content of which is incorporated herein by reference. This reference provides for a 2D structure that can be folded into a T-shaped, or Pi-shaped, 3D structure, so called because the folded portion of the preform may produce either one or two legs or flanges (for T- and Pi-shapes, respectively) generally perpendicular to the base or parent material. This is accomplished by adjusting the length of fibers during weaving to prevent the above mentioned dimples and buckles at the site of the fold. In the weaving process, some fibers are woven too long, and others woven too short, in the region of the fold. The short and long fibers are then equalized in length as the preform is folded, yielding a smooth transition at the fold.
The benefit of folded preforms is the strength of the joint between the panel to be reinforced and the reinforcing panel. As they are woven together, the panels share reinforcing material and in the final composite, matrix material, creating a unitary construction. The juncture between the integrally woven reinforcement flange or leg and the parent material or base is no longer the weak link, relying solely upon the strength of the adhesive for the strength of the joint, as in the prior art reinforcements. Instead, the fibers of the preform integrally weave the legs and the base together.
Frequently, however, complex shapes, such as curves or sharp corners, require reinforcement. Folded T- or Pi-shaped reinforcements require darting of the legs in order to accommodate a curved or angled surface. As the flange material of a folded preform assumes a curved or angled shape, the length of the curved surface necessarily varies from the inside of the curvature to the outside. The arc length of outside of the curvature, the surface with the larger radius when curved, increases, while on the inside of the curvature, the arc length decreases. The legs of typical folded preforms cannot change length as required to accommodate a curved or angled surface. To accommodate a curved or angled surface, the legs must be darted, that is they must be cut in order to allow the leg to conform to the changed arc length.
Typically, the cut is along the localized radius of curvature, but other, non-radial cuts may also be used to accommodate the change in length. To allow for the decreased length on the inside of a curved preform, the leg is cut and the cut edges allowed to overlap, or the excess material is removed. Similarly, to accommodate the increased length on the outside of the curvature, the leg is cut, resulting in a triangular gap between cut edges of the leg. In either configuration, the darting breaks the continuity of the reinforcing material in each leg. Darting the legs of a 3D T- or Pi-preform can seriously degrade the load carrying capabilities of the preform, because darting involved cutting the fibers that provide the primary load path around the corner.